Improving a software defined radio with a few bits of wire

Impressed by the recent advances in the software defined radio scene, [Jason] picked up a $20 USB TV tuner dongle to check out his local airwaves. Unfortunately, the antenna included with the little USB dongle is terrible at receiving any signal other than broadcast TV. [Jason] wanted to improve his reception, so he got some wire and made his own discone antenna.

The discone antenna is ideally suited for [Jason]’s setup – properly constructed, it’s able to receive over the entire 64 to 1700 MHz band the RTL-SDR dongle is able to read. To construct his antenna, [Jason] checked out [VE3SQB]’s list of antenna design programs, got the dimensions of his antenna, and set to work attaching wire to PVC pipe.

The antenna is a massive improvement over the stock antenna included with the TV tuner dongle. After mounting his discone at the far end of his back yard, [Jason] started picking up a few blips from the transponders of passing aircraft.

It’s hard for physics to become outdated… antenna theory hasn’t changed. Perhaps people’s ability/willingness to slog through the maths has decreased (and software is faster, for sure), but the ARRL antenna handbook is certainly not outdated.

Do note that a Discone like this is very prone to overloading the RTL2832U based dongles. Makes it harder to listen to signals in other bands when the antenna is also grabbing the 88-108MHz radio stations and overloading the tuner. It is a great idea to filter out the broadcast frequencies.

Also, many of the dongles are missing the static protection diodes and building and attaching one of these can quite easily break your dongle.

I think Amazed meant an RF limiter when he says static protection diodes. RF limiters for wideband reception are usually implemented using PIN diodes. Small signal diodes like 1N914 (US) or 1N4148 (EU) are not useful because they have too much parasitic capacitance (<4pF) and they create unwanted inter-modulation products. The Avago website has several application notes pertaining limiters – look for AN1050, AN5438 and AN5443.

As you can see in fig. 4 of the aforementioned document, the discone’s beam is ‘lifted’ (Brewster angle) in the elevation plane (same as other ground-plane verticals). I think it is for this reason, professional aerial makers like Agilent and EMCO, prefer to market the bi-conical variant.

Yes, the government trunking still uses narrow band FM in my area at about 160MHz. I can get the police, ambulance and other services loud and clear with the RTLSDR dongle and the HDSDR program in Windows.